Color transform for video coding

ABSTRACT

A method of video decoding performed in a video decoder includes receiving a syntax element from a bitstream of a coded video. The syntax element can indicate that residual blocks of a current coding unit (CU) are processed with a color space conversion. The residual blocks of the current CU can include a luma residual block, a first chroma residual block, and a second chroma residual block. In response to receiving the syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, one of two options of color space conversion equations can be selected. An inverse color space conversion using the selected option of color space conversion equations can be applied to the residual blocks of the current CU to generate modified versions of the residual blocks of the current CU.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 62/913,486, “Improved Color Transform InVVC” filed on Oct. 10, 2019, which is incorporated by reference hereinin its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

The intra prediction modes used in HEVC are illustrated in FIG. 1B. InHEVC, there are total 35 intra prediction modes, among which mode 10 ishorizontal mode, mode 26 is vertical mode, and mode 2, mode 18 and mode34 are diagonal modes. The intra prediction modes are signalled by threemost probable modes (MPMs) and 32 remaining modes.

FIG. 1C illustrates the intra prediction modes used in VVC. In VVC,there are total 95 intra prediction modes as shown in FIG. 1C, wheremode 18 is the horizontal mode, mode 50 is the vertical mode, and mode2, mode 34 and mode 66 are diagonal modes. Modes −1˜−14 and Modes 67˜80are called Wide-Angle Intra Prediction (WAIP) modes.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingMPMs, and similar techniques. In all cases, however, there can becertain directions that are statistically less likely to occur in videocontent than certain other directions. As the goal of video compressionis the reduction of redundancy, those less likely directions will, in awell working video coding technology, be represented by a larger numberof bits than more likely directions.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1D, a current block (110) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using. The order of forming acandidate list may be A0→B0→B1→A1→B2.

SUMMARY

Aspects of the disclosure provide a method of video decoding performedin a video decoder. A syntax element can be received from a bitstream ofa coded video. The syntax element can indicate that residual blocks of acurrent coding unit (CU) are processed with a color space conversion.The residual blocks of the current CU can include a luma residual block,a first chroma residual block, and a second chroma residual block. Inresponse to receiving the syntax element indicating that the residualblocks of the current CU are processed with the color space conversion,one of two options of color space conversion equations can be selected.An inverse color space conversion using the selected option of colorspace conversion equations can be applied to the residual blocks of thecurrent CU to generate modified versions of the residual blocks of thecurrent CU.

In an embodiment, the two options of color space conversion equationsinclude a first option of color space conversion equations including:tmp=rY[x][y]−(rCb[x][y]>>1),rY[x][y]=tmp+rCb[x][y],rCb[x][y]=tmp−(rCr[x][y]>>1), andrCr[x][y]=rCb[x][y]+rCr[x][y],and a second option of color space conversion equations including:tmp=rY[x][y]−rCb[x][y],rY[x][y]=rY[x][y]+rCb[x][y],rCb[x][y]=tmp−rCr[x][y], andrCr[x][y]=tmp+rCr[x][y].Inputs to each option of color space conversion equations include anarray of luma residual samples of the luma residual block with elementsrY[x][y], an array of chroma residual samples of the first chromaresidual block with elements rCb[x][y], and an array of chroma residualsamples of the second chroma residual block with elements rCb[x][y].Outputs from each option of color space conversion equations include amodified array of luma residual samples of the luma residual block withelements rY[x][y], a modified array of chroma residual samples of thefirst chroma residual block with elements rCb[x][y], and a modifiedarray of chroma residual samples of the second chroma residual blockwith elements rCb[x][y].

In an embodiment, the first option of color space conversion equationsis selected when a transform skip flag corresponding to each of theresidual blocks of the current CU has a value of 1. In an embodiment,the second option of color space conversion equations is selected whenat least one of transform skip flags corresponding to the residualblocks of the current CU has a value of 0.

In an embodiment, the first option of color space conversion equationsis selected when a transform skip flag corresponding to each of theresidual blocks of the current has a value of 1 and a quantizationparameter (QP) corresponding to each of the residual blocks of thecurrent CU has a value of 4. In an embodiment, the second option ofcolor space conversion equations is selected when at least one of QPscorresponding to the residual blocks of the current CU has a value notequal to 4.

In an embodiment, a syntax element is received that indicates only thefirst option of color space conversion equations is applied. The syntaxelement is one of a slice level syntax element, a picture level syntaxelement, or a sequence level syntax element. In an embodiment, a syntaxelement is received that indicates only the first option of color spaceconversion equations is applied regardless of a value of a flagindicating whether transform and quantization are bypassed for residualsof a CU. The syntax element is one of a slice level syntax element, apicture level syntax element, or a sequence level syntax element.

In an embodiment, one of two options of color space conversion equationsis selected according to a value of a transform skip flag correspondingto each of the residual blocks of the current CU. In an embodiment, oneof two options of color space conversion equations is elected accordingto a variable indicating whether a block-based differential pulse codemodulation (BDPCM) is applied to the current CU.

An embodiment of the method can further include determining no colorspace conversion is applied to residual blocks of a CU when a bit depthof luma samples of the CU is different from a bit depth of chromasamples of the CU. An embodiment of the method can further includedetermining no color space conversion is applied to residual blocks of aCU when a bit depth of luma samples of the CU is different from a bitdepth of chroma samples of the CU, and a transform skip flagcorresponding to each of residual blocks of the CU has a value of 1 or0. An embodiment of the method can further include determining no colorspace conversion is applied to residual blocks of a CU when a bit depthof luma samples of the CU is different from a bit depth of chromasamples of the CU, a QP corresponding to each of residual blocks of theCU has a value of 4, and a transform skip flag corresponding to each ofthe residual blocks of the CU has a value of 1 or 0.

Aspects of the disclosure provide an apparatus of video decoding. Theapparatus can includes circuitry configured to receive a syntax elementfrom a bitstream of a coded video indicating that residual blocks of acurrent CU are processed with a color space conversion. The residualblocks of the current CU can include a luma residual block, a firstchroma residual block, and a second chroma residual block. In responseto receiving the syntax element indicating that the residual blocks ofthe current CU are processed with the color space conversion, one of twooptions of color space conversion equations can be selected. An inversecolor space conversion using the selected option of color spaceconversion equations can be applied to the residual blocks of thecurrent CU to generate modified versions of the residual blocks of thecurrent CU.

Aspects of the disclosure provide a non-transitory computer-readablemedium storing instructions that, when executed by a processor, causethe processor to perform the method of video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes;

FIG. 1B is an illustration of exemplary intra prediction directions.

FIG. 1C is an illustration of exemplary intra prediction directions.

FIG. 1D is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8A shows block partitioning in accordance with an embodiment.

FIG. 8B shows a block partitioning tree in accordance with anembodiment.

FIG. 9A shows a vertical center-side ternary tree partitioning inaccordance with an embodiment.

FIG. 9B shows a horizontal center-side ternary tree partitioning inaccordance with an embodiment.

FIGS. 10A-10D illustrate different chroma formats in accordance withvarious embodiments.

FIG. 11 illustrates an example encoder in accordance with an embodiment.

FIG. 12 illustrates an example decoder in accordance with an embodiment.

FIG. 13 illustrates a straight line between a minimum and maximum lumavalue in accordance with an embodiment.

FIGS. 14A and 14B illustrate locations of samples used for derivation ofα and β in LT_CCLM in accordance with an embodiment.

FIGS. 15A and 15B illustrate locations of the samples used for thederivation of α and β in T_CCLM in accordance with an embodiment.

FIGS. 16A and 16B illustrate locations of the samples used for thederivation of α and β in L_CCLM in accordance with an embodiment.

FIG. 17 illustrates an example of classifying neighboring samples intotwo groups in accordance with an embodiment.

FIG. 18 is a schematic illustration of an encoder and a decoder inaccordance with an embodiment.

FIG. 19 is an illustration of an embodiment of a process performed by adecoder.

FIG. 20 is a schematic illustration of a computer system in accordancewith an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Encoder and Decoder System

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. Ina videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Tree-Structure Based Block Partitioning

According to some embodiments, a CTU is split into CUs by using a quadtree binary tree (QTBT) structure denoted as a coding tree to adapt tovarious local characteristics of individual blocks included in the CUs.The decision whether to code a picture area using inter-picture(temporal) or intra-picture (spatial) prediction may be performed at theCU level. Each CU may be further split into one, two or four PUsaccording to a PU splitting type. In some embodiments, inside one PU,the same prediction process is applied and the relevant information istransmitted to the decoder on a PU basis. After obtaining the residualblock by applying the prediction process based on the PU splitting type,a CU may be partitioned into TUs according to another quad treestructure similar to the quad tree structure used for the coding treefor the CTU. In some other embodiments, a PU contains only one TU thathas the same shape as the PU.

The coding tree for the CTU may include multiple partition typesincluding CU, PU, and TU. In some embodiments, a CU or a TU is only asquare shape, while a PU may be square or rectangular shape for an interpredicted block. In other embodiments, rectangular shaped CUs, PUs, andTUs are permitted. At a picture boundary, an implicit quad tree splitmay be applied so that a block will keep quad tree splitting until thesize of the split block fits the picture boundary. According to someembodiments, an implicit split means that a split flag is not signaledbut implied instead. For example, implicit QT means only a QT split isallowed for a picture boundary block. As such, the split flag is notsignaled at the picture boundary. As an another example, when only a BTsplit is allowed at the picture boundary, the implicit split is thebinary split. In some embodiments, when both QT and BT are allowed atthe picture boundary, there is no implicit split, and the split methodis explicitly signaled.

According to some embodiments, the QTBT structure does not includemultiple partition types (e.g., QTBT does not include the separation ofthe CU, PU and TU), and supports more flexibility for CU partitionshapes. For example, in the QTBT block structure, a CU may have either asquare or rectangular shape. FIG. 8A illustrates an example CTU (800)that is partitioned by the QTBT structure. For example, the CTU (800) ispartitioned into four equal sized sub-CUs (A), (B), (C), and (D). FIG.8B illustrates a corresponding coding tree that illustrates branchescorresponding to sub-CUs (A), (B), (C), and (D). The solid linesindicate quad tree splitting, and the dotted lines indicate binary treesplitting. The binary tree structure may include two splitting types:(i) symmetric horizontal splitting and (ii) symmetric verticalsplitting. In each splitting (i.e., non-leaf) node of the binary tree,one flag may be signalled to indicate which splitting type (e.g.,horizontal or vertical) is used, where 0 indicates horizontal splittingand 1 indicates vertical splitting or vice versa. For the quad treesplitting, the splitting type is not indicated since quad tree splittingsplits a block both horizontally and vertically to produce 4 sub-blockswith an equal size.

As illustrated in FIGS. 8A and 8B, the sub-CU (A) is first partitionedinto two sub-blocks by a vertical split, where the left sub-block ispartitioned again by another vertical split. The sub-CU (B) is furtherpartitioned by a horizontal split. The sub-CU (C) is further partitionedby another quad split partition. The upper left sub-block of sub-CU (C)is partitioned by a vertical split, and subsequently partitioned by ahorizontal split. Furthermore, the lower right sub-block of sub-CU (C)is partitioned by a horizontal split. The upper right and lower leftsub-blocks of sub-CU (C) are not further partitioned. The sub-CU (D) isnot partitioned further and thus, does not include any additional leafnodes in the coding tree below the “D” branch.

The binary tree leaf nodes may be referred to as CUs, where the binarysplitting may be used for prediction and transform processing withoutany further partitioning, which means that the CU, PU, and TU have thesame block size in the QTBT coding block structure. A CU may includecoding blocks (CBs) of different color components. For example, one CUmay contain one luma CB and two chroma CBs in the case of P and B slicesof a 4:2:0 chroma format, and sometimes contain a CB of a singlecomponent (e.g., one CU contains only one luma CB or just two chroma CBsin the case of Intra-pictures or I slices). In some embodiments, inintra-pictures or I-slices, the TU width or height is constrained to notexceed a given limit (e.g., 64 for luma and 32 for chroma). If the CBwidth or height is larger than the limit, then the TU is further splituntil the TU's size does not exceed the limit.

According to some embodiments, the QTBT partitioning scheme includes thefollowing parameters:

CTU size: the root node size of a quad tree

MinQTSize: the minimum allowed quad tree leaf node size

MaxBTSize: the maximum allowed binary tree root node size

MaxBTDepth: the maximum allowed binary tree depth

MinBTSize: the minimum allowed binary tree leaf node size

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 luma samples with two corresponding 64×64 blocks of chromasamples, the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64,the MinBTSize (for both width and height) is set as 4×4, and theMaxBTDepth is set as 4. The QTBT partitioning structure is applied tothe CTU first to generate quad tree leaf nodes. The quad tree leaf nodesmay have a size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., theCTU size). If the leaf quad tree node is 128×128, the leaf quad treenode will not be further split by the binary tree since the size exceedsthe MaxBTSize (i.e., 64×64). Otherwise, the leaf quad tree node may befurther partitioned by the binary tree. Therefore, the quad tree leafnode is also the root node for the binary tree and the quad tree leafhas the binary tree depth as 0. When the binary tree depth reaches theMaxBTDepth (e.g., 4), no further splitting is performed. When the binarytree node has width equal to the MinBTSize (e.g., 4), no furtherhorizontal splitting is performed. Similarly, when the binary tree nodehas a height equal to MinBTSize, no further vertical splitting isperformed. The leaf nodes of the binary tree are further processed byprediction and transform processing without any further partitioning. Insome embodiments, the maximum CTU size is 256×256 luma samples.

The QTBT partition structure may further support the ability for theluma and chroma components to each have separate QTBT structures. Forexample, for P and B slices, the luma and chroma CTBs in one CTU mayshare the same QTBT structure. However, for I slices, the luma CTB ispartitioned into CUs by a QTBT structure, and the chroma CTBs arepartitioned into chroma CUs by another QTBT structure. Therefore, inthis example, a CU in an I slice contains a coding block of the lumacomponent or coding blocks of two chroma components, and a CU in a P orB slice contains coding blocks of all three color components.

In some embodiments, inter prediction for small blocks is restricted toreduce the memory access requirements of motion compensation, such thatbi-prediction is not supported for 4×8 and 8×4 blocks, and interprediction is not supported for 4×4 blocks. In other embodiments, theQTBT partition scheme does not include these restrictions.

According to some embodiments, a Multi-type-tree (MTT) structureincludes (i) quad tree splitting, (ii) binary tree splitting, and (iii)horizontal and vertical center-side ternary trees. FIG. 9A illustratesan embodiment of a vertical center-side ternary tree and FIG. 9Billustrates an example of a horizontal center-side ternary tree.Compared to the QTBT structure, MTT can be a more flexible treestructure since additional structures are permitted.

Ternary tree partitioning includes significantly advantageous featuressuch as providing a complement to quad tree and binary tree partitioningwhere ternary tree partitioning is able to capture objects which arelocated in a block center, whereas quad tree and binary tree split alongthe block center. As another advantage of ternary tree partitioning, thewidth and height of the partitions of the proposed ternary trees are apower of 2 so that no additional transforms are needed. A two-level treeprovides the advantage of complexity reduction. As an example, thecomplexity of traversing a tree is T^(D), where T denotes the number ofsplit types, and D is the depth of tree.

III. YUV Formats

There are different YUV formats or chroma formats (chroma samplingformats), which are shown in FIGS. 10A-10D. Each chroma format maydefine a different sampling or down-sampling grid of different colorcomponents.

IV. Color Formats

The color of video samples may be represented in different color formatsor color spaces (e.g., YCbCr or RGB). In the RGB format, the threecomponents (i.e., R, G, and B) have strong correlations, which resultsin statistical redundancy among the three color components. A colorrepresentation of video samples may be converted into a different colorspace using linear transforms. Such an operation or process can bereferred to as a color space conversion.

Converting a RGB color space to a YUV color space may be performed asfollows:Y=((66*R+129*G+25*B+128)>>8)+16  Eq. (1):U=((−38*R−74*G+112*B+128)>>8)+128  Eq. (2):V=((112*R−94*G−18*B+128)>>8)+128  Eq. (3):

In an alternative form, converting a RGB color space to a YUV colorspace may be performed as follows:Y=round(0.256788*R+0.504129*G+0.097906*B)+16  Eq. (4):U=round(−0.148223*R−0.290993*G+0.439216*B)+128  Eq. (5):V=round(0.439216*R−0.367788*G−0.071427*B)+128  Eq. (6):

V. Adaptive Color Transform

The embodiments disclosed herein may be used separately or combined inany order. Further, each of the methods, encoder, and decoder accordingto the embodiments of the present disclosure may be implemented byprocessing circuitry (e.g., one or more processors or one or moreintegrated circuits). In one example, the one or more processors executea program that is stored in a non-transitory computer-readable medium.According to embodiments of the present disclosure, the term block maybe interpreted as a prediction block, a coding block, or a coding unit(i.e., CU).

According to embodiments of the present disclosure, the term lumacomponent may refer to any color component that is coded as the firstcomponent in coding order (e.g., (R)ed or (G)reen color component).Furthermore, according to embodiments of the present disclosure, theterm chroma component may refer to any color component that is not codedas the first component in coding order. For example, the second or thirdcomponent can be referred to as a chroma component.

In some embodiments, for efficient coding of RGB video content, anin-loop color transform (also referred to as color space conversion) canbe employed to handle different characteristics of image blocks. As thecolor transform can be used adaptively for different CUs, the colortransform as a coding tool can be referred to as adaptive colortransform (ACT). The ACT can operate in the residue domain in someexamples. A CU-level flag may be signaled to indicate the usage of ACT.

For example, much screen content is captured in the RGB color space. Foran image block in the RGB color space, usually, there can be strongcorrelation among different color components such that a color spaceconversion is useful for removing inter-color component redundancy.However, for screen content, there may exist many image blockscontaining different features having very saturated colors, which leadsto less correlation among color components. For those blocks, codingdirectly in the RGB color space may be more effective. Accordingly,color space conversion can be adaptively applied to image blocks withdifferent characteristics.

In an example, the forward ACT color transform (e.g., transformperformed at encoder) may be performed as follows to convert an imageblock in the RGB color space to a YCoCg color space:

$\begin{matrix}{\begin{bmatrix}Y \\C_{g} \\C_{o}\end{bmatrix} = {{\frac{1}{4}\begin{bmatrix}1 & 2 & 1 \\{- 1} & 2 & {- 1} \\2 & 0 & {- 2}\end{bmatrix}} \times \begin{bmatrix}R \\G \\B\end{bmatrix}}} & {{Eq}.\mspace{14mu}(7)}\end{matrix}$

The backward ACT color transform (e.g., inverse transform performed atdecoder) may be performed as follows:

$\begin{matrix}{\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}1 & {- 1} & 1 \\1 & 1 & 0 \\1 & {- 1} & {- 1}\end{bmatrix} \times \begin{bmatrix}Y \\C_{g} \\C_{o}\end{bmatrix}}} & {{Eq}.\mspace{14mu}(8)}\end{matrix}$

FIG. 11 illustrates an example encoder (1100) for performing a colorspace transform. In FIG. 11, prediction is performed prior to the colorspace transform being applied. For example, inter prediction or intraprediction is performed on a current block to produce a residual signal.The residual signal is provided to a forward color space transform unit(1102) to perform a forward transform such as the transform in Eq. (7).The output of the forward color space transform is provided to a crosscomponent prediction (CCP) unit (1102). The output of the (CCP) unit(1102) is provided to a transform (T) unit (1106) to perform a transformsuch as a type of discrete cosine transform (DCT) to generate transformcoefficients. The output of the transform unit (1106) is provided to aquantizer (Q) (1108) to produce quantized coefficients. The quantizedcoefficients are provided to an entropy coder unit (1110) to convertbins of the quantized coefficients to bits in a bitstream. The entropycoder unit (1110) may receive intra prediction mode or motion vector(MV) information associated with the current block, and entropy encodingthat information into the bitstream.

The encoder (1100) may also include the components to reconstruct aresidual signal. For example, the quantized coefficients may be providedto an inverse quantizer (IQ) unit (1112). The output of the inversequantizer unit (IQ) may be provided to an inverse transform (IT) unit(1114). The output of the inverse transform (IT) unit (1114) may beprovided to an inverse CCP unit (1116). The output of the inverse CCPunit (1116) may be provided to an inverse color space transform unit(1118) where an inverse color transform such as the transformillustrated in Eq. (8) may be performed to generate the reconstructedresidual signal.

FIG. 12 illustrates an example decoder (1200) for converting a bitstreamto a residual signal. The bitstream illustrated in FIG. 12 may be thebitstream produced by entropy coder (1110) in the FIG. 11 example. Thebitstream may be provided to an entropy decoder unit (1202). The outputof an entropy decoder unit (1202) may be provided to an inversequantizer (IQ) unit (1204). The output of the inverse quantizer unit(IQ) (1204) may be provided to an inverse transform (IT) unit (1206).The output of the inverse transform (IT) unit (1206) may be provided toan inverse CCP unit (1208). The output of the inverse CCP unit (1208)may be provided to an inverse color space transform unit (1210) where aninverse color transform such as the transform illustrated in Eq. (8) maybe performed to produce the residual signal. Intra prediction or interprediction may be performed to generate a prediction block which iscombined with the residual signal to reconstruct a current block. Theunits disclosed in FIGS. 11 and 12 may be implemented in softwareexecuted by a processor or by circuitry such as a specialized integratedcircuit designed to perform the function of each unit.

In the examples of FIGS. 11 and 12, the CCP operation can be optionallyperformed in spatial residual domain. For example, the residual of thesecond or third color component (also referred to as chroma components)can be predicted from the residual of the first color component (alsoreferred to as luma components) in CCP. When the ACT is not used, theCCP can significantly improve coding efficiency when coding videos havethe RGB color format. When the ACT is used, the CCP can still be used incombination with the ACT to improve the coding efficiency for videos inthe RGB color space.

In the Examples of FIGS. 11 and 12, the transform operation can beoptionally performed, for example, depending on characteristics of CUs.For example, for certain type of screen content, avoiding usage of thetransform operation may have a better coding performance than using thetransform. For example, a CU level transform skip flag can be signaledfor indicating whether a transform is skipped for the CU. Similarly, thequantization operation may also be optional. For example, when losslessmode is configured for certain types of video content (medical images,wireless local screen sharing), the transform and the quantization canbe skipped for certain CUs. For example, a CU level CU transform andquantization skip flag can be signaled for indicating a lossless modebeing applied.

In some embodiments, unless a maximum transform size is smaller than thewidth or height of one coding unit (CU), one CU leaf node is also usedas the unit of transform processing. In an example, an ACT flag issignaled for one CU to select the color space for coding its residuals.In addition, in an example, for inter and intra block copy (IBC) CUs,the ACT is enabled when there is at least one non-zero coefficient inthe CU. For intra CUs, the ACT is enabled when chroma components selectthe same intra prediction mode of luma component, i.e., DM mode. In anembodiment, to compensate a dynamic range change of residuals signalsbefore and after color transform, QP adjustments, for example, of (−5,−5, −3), are applied to the transform residuals.

As the forward and inverse color transforms need to access the residualsof all three components, in some embodiments, the ACT is disabled forsome scenarios where not all residuals of three components areavailable. For example, when separate-tree partition is employed, lumaand chroma samples inside one CTU are partitioned by different treestructures. This results in that the CUs in the luma-tree only containsluma component and the CUs in the chroma-tree only contains two chromacomponents. When intra sub-partition prediction (ISP) is employed, theISP sub-partition is applied to luma while chroma signals are codedwithout splitting. Except the last ISP sub-partitions, the othersub-partitions only contain luma component. Accordingly, the ACT can bedisabled for separate-tree (dual tree) partition and ISP.

VI. Cross-Component Linear Model (CCLM)

For the chroma component of an intra PU, the encoder may select the bestchroma prediction modes among 8 modes including Planar, DC, Horizontal,Vertical, a direct copy of the intra prediction mode (DM) from the lumacomponent, Left and Top Cross-component Linear Mode (LT_CCLM), LeftCross-component Linear Mode (L_CCLM), and Top Cross-component LinearMode (T_CCLM). LT_CCLM, L_CCLM, and T_CCLM can be categorized into thegroup of Cross-component Linear Mode (CCLM). The difference betweenthese 3 modes is that different regions of neighboring samples may beused for deriving the parameters α and β. For LT_CCLM, both the left andtop neighboring samples may be used to derive the parameters α and β.For L_CCLM, in some examples, only the left neighboring samples are usedto derive the parameters α and β. For T_CCLM, in some examples, only thetop neighboring samples are used to derive the parameters α and β.

Cross-Component Linear Model (CCLM) prediction modes may be used toreduce a cross-component redundancy, in which the chroma samples arepredicted based on the reconstructed luma samples of the same CU byusing an example linear model as follows:pred_(C)(i,j)=α·rec_(L)′(i,j)+β,  Eq. (9):where pred_(C)(i,j) represents the predicted chroma samples in a CU andrec_(L)(i,j) represents the downsampled reconstructed luma samples ofthe same CU. Parameters α and β may be derived by a straight lineequation, which may also be referred to as a max-min method. Since thiscomputation process may be performed as part of the decoding process,not just as an encoder search operation, no syntax may be used to conveythe α and β values.

For chroma 4:2:0 format, CCLM prediction may apply a six-tapinterpolation filter to get the down-sampled luma sample correspondingto a chroma sample as shown in FIGS. 10B-10D. In a formula way, adown-sampled luma sample Rec′L[x, y] is calculated from reconstructedluma samples as follows. The down-sampled luma samples may be used tofind the maximum and minimum sample points. The 2 points (a pair of Lumaand Chroma) (A, B) may be the minimum and maximum values inside the setof neighboring Luma samples as depicted in FIG. 13.

In FIG. 13, the linear model parameters α and β may be obtainedaccording to the following equations:

$\begin{matrix}{\alpha = \frac{y_{B} - y_{A}}{x_{B} - x_{A}}} & {{Eq}.\mspace{14mu}(10)} \\{\beta = {y_{A} - {\alpha\; x_{A}}}} & {{Eq}.\mspace{14mu}(11)}\end{matrix}$

A division operation is advantageously avoided by using a multiplicationand a shift operation in an example. One Look-up Table (LUT) may be usedto store the pre-calculated values, and the absolute difference valuesbetween maximum and minimum luma samples may be used to specify theentry index of the LUT. The size of the LUT may be 512 in an example.

FIGS. 14A and 14B illustrate example locations of the samples used forthe derivation of α and β in LT_CCLM. In T_CCLM mode, in some examples,only the above neighboring samples (including 2*W samples) are used tocalculate the linear model coefficients. FIGS. 15A and 15B illustrateexample locations of the samples used for the derivation of α and β inT_CCLM.

In L_CCLM mode, in some examples, only left neighboring samples(including 2*H samples) are used to calculate the linear modelcoefficients. FIGS. 16A and 16B illustrate example locations of thesamples used for the derivation of α and β in L_CCLM.

The CCLM prediction mode may also include prediction between the twochroma components (i.e., the Cr component is predicted from the Cbcomponent). Instead of using the reconstructed sample signal, the CCLMCb-to-Cr prediction may be applied in the residual domain. The CCLMCb-to-Cr prediction may be implemented by adding a weightedreconstructed Cb residual to the original Cr intra prediction to formthe final Cr prediction:pred*_(Cr)(i,j)=pred_(Cr)(i,j)+α·resi_(Cb)′(i,j)  Eq. (12):

The CCLM luma-to-chroma prediction mode may be added as one additionalchroma intra prediction mode. At the encoder side, one more ratedistortion (RD) cost checks for the chroma components are added forselecting the chroma intra prediction mode. When intra prediction modesother than the CCLM luma-to-chroma prediction mode is used for thechroma components of a CU, CCLM Cb-to-Cr prediction is used for Crcomponent prediction.

Multiple Model CCLM (MMLM) is another extension of CCLM, where there canbe more than one model (e.g., two or more models). In MMLM, neighboringluma samples and neighboring chroma samples of the current block may beclassified into two groups, where each group may be used as a trainingset to derive a linear model (i.e., a particular α and β are derived fora particular group). Furthermore, the samples of the current luma blockmay also be classified based on the same rule for the classification ofneighboring luma samples.

FIG. 17 shows an example of classifying the neighbouring samples intotwo groups. The threshold illustrated in FIG. 17 may be calculated asthe average value of the neighbouring reconstructed luma samples. Aneighbouring sample with Rec′L[x,y]<=Threshold is classified into group1; while a neighbouring sample with Rec′L[x,y]>Threshold is classifiedinto group 2.

$\begin{matrix}\left\{ \begin{matrix}{{{Pred}_{C}\left\lbrack {x,y} \right\rbrack} = {{{\alpha_{1} \times {{Rec}_{L}^{\prime}\left\lbrack {x,y} \right\rbrack}} + {\beta_{1}\mspace{14mu}{if}\mspace{14mu}{{Rec}_{L}^{\prime}\left\lbrack {x,y} \right\rbrack}}} \leq {Threshold}}} \\{{{Pred}_{C}\left\lbrack {x,y} \right\rbrack} = {{{\alpha_{2} \times {{Rec}_{L}^{\prime}\left\lbrack {x,y} \right\rbrack}} + {\beta_{2}\mspace{14mu}{if}\mspace{14mu}{{Rec}_{L}^{\prime}\left\lbrack {x,y} \right\rbrack}}} > {Threshold}}}\end{matrix} \right. & {{Eq}.\mspace{14mu}(13)}\end{matrix}$

VII. Examples of ACT Applied for Prediction Process

According to some embodiments, a color transform (such as ACT) can beapplied before a prediction process is performed at the encoder, andafter the reconstruction process is performed at the decoder. At theencoder, the ACT may be performed before prediction (e.g., interprediction, intra prediction), and the reference samples and inputoriginal samples may both be mapped to a different color space if ACT isapplied for a current CU. For pixel reconstruction at the decoder, ifACT is applied to a block under reconstruction, the reference samplesmay be mapped to the alternative color space before being used forprediction, and reconstructed samples may then be mapped back to theoriginal color space.

FIG. 18 illustrates an embodiment of the encoder and decoder processesusing ACT. The units disclosed in FIG. 18 may be implemented in softwareexecuted by a processor or by circuitry such as a specialized integratedcircuit designed to perform the function of each unit disclosed in FIG.18. At the encoder, ACT units (1800) and (1804) perform an ACT transformon both a reference signal and an input signal, respectively. The ACTtransform performed at the encoder by ACT units (1800) and (1804) may bethe ACT transform disclosed in Eq. (7). The output of ACT (1800) isprovided to a prediction (P) unit (1802). Furthermore, the referencesignal is provided to a prediction (P) unit (1806). The prediction (P)units (1802) and (1806) may perform inter prediction or intraprediction. A transform (T) unit 1808 receives one of (i) a differencebetween the output of the prediction (P) unit (1802) and the output ofthe ACT unit (1804) and (ii) a difference between an output of theprediction (P) unit (1806) and the input signal. The transform (T) unit(1808) may perform a transform operation such as a discrete cosinetransform (DCT). The output of the transform (T) unit 1808 is providedto a quantizer unit (Q) (1810) to perform a quantization operation toproduce a set of coefficients.

At the decoder, an inverse quantizer (IQ) unit (1812) receivescoefficients to perform an inverse quantization process. The output ofthe inverse quantizer (IQ) unit (1812) is provided to an inversetransform (IT) unit (1814) to perform an inverse transform. An ACT unit(1820) receives a sum of an output of a prediction (P) unit (1818) andthe output of the inverse transform (IT) (1814) unit. An ACT unit (1816)receives a reference signal and perform an ACT to generate an input tothe prediction (P) unit (1818). The ACT unit (1816) may perform a colortransform such as the color transform disclosed in Eq. (7). The ACT unit(1820) may perform an inverse color transform such as the inverse colortransform disclosed in Eq. (8). The prediction (P) units (1818) and(1822) may perform inter prediction or intra prediction. Thereconstructed original signal is provided by the output of the ACT unit(1820), or by combination of the outputs of the prediction unit (1822)and the inverse transform (IT) unit (1814).

VIII. Modified Color Transform Equations

According to some embodiments, in an ACT process, the 2nd and 3rd colorcomponents (chroma components) can further be offset by a constant cafter and before the color transform for forward and backward transform,respectively. Eq. (14) illustrates the modified forward transform, andEq. (15) illustrates the modified backward (i.e., inverse) transform.

$\begin{matrix}{\begin{bmatrix}Y \\C_{g} \\C_{o}\end{bmatrix} = {{{\frac{1}{4}\begin{bmatrix}1 & 2 & 2 \\{- 1} & 2 & {- 1} \\2 & 0 & {- 2}\end{bmatrix}} \times \begin{bmatrix}R \\G \\B\end{bmatrix}} + \begin{bmatrix}0 \\c \\c\end{bmatrix}}} & {{Eq}.\mspace{14mu}(15)} \\{\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}1 & {- 1} & 1 \\1 & 1 & 0 \\1 & {- 1} & {- 1}\end{bmatrix} \times \begin{bmatrix}Y \\{C_{g} - c} \\{C_{o} - c}\end{bmatrix}}} & {{Eq}.\mspace{14mu}(16)}\end{matrix}$

In some embodiments, the constant c is derived as 1<<(bitDepth−1), wherebitDepth refers to a bit-depth of an input sample.

IX. Examples of Interactions Between ACT and Other Coding Tools

In some embodiments, to enable in-loop color transform for efficientcoding of input video with RGB format, interactions between colortransform (in residual domain or before prediction) and several codingtools needs to be handled. Examples of such coding tools includecross-component linear model (CCLM) and dual tree partitioning.Embodiments of the present disclosure provide the advantageous featuresof handling color transform with consideration of certain coding tools.

In some embodiments, a color transform is only applied when differentcolor components are coded using a same transform unit partitioningtree. In one embodiment, when dual tree is applied on an intra slice,the color transform is applied for an inter slice only.

In some embodiments, when a color transform (e.g., in spatial residualdomain) is applied, the CCLM mode is not applied or signaled since thegeneration of residual samples from one component depends on thereconstruction of another component. In another embodiment, when theCCLM mode is used, the color transform (e.g., in residual domain) is notapplied or signaled. In one embodiment, when the color transform isapplied on intra residual samples, the CCLM mode is not applied orsignaled since the generation of residual samples from one componentdepends on the reconstruction of another component. In one embodiment,when the color transform is applied on a residual sample and CCLM modeis used, the color transform (e.g., before prediction) is not applied orsignaled.

In some embodiments, a color transform is signaled for each CTU which isthe largest coding unit (CU). In some embodiments, a color transform issignaled and applied only for intra coded blocks, or only for intercoded blocks. In some embodiments, when a color transform is applied,the dual tree is not applied (i.e., different color components share thesame transform unit partitioning).

In some embodiments, for a current CU, when a bit depth of samples of aluma sample array, denoted by BitDepthY, is different from a bit depthof samples of a chroma sample array, denoted by BitDepth C, colortransform (e.g., in spatial residual domain, or before prediction) isnot used on the CU. For example, the current CU can be partitioned froma picture having a 4:4:4 chroma format (RGB color format), however, theluma component and the chroma component are represented with differentbit depths. For example, the luma component has a bit depth of 10 bitswhile the chroma component has a bit depth of 8 bits. The colortransform by applying the various color transform equations (or colorspace conversion equations) disclosed herein may be ineffective forreducing statistic dependency among the luma and chroma components. Insuch a scenario, a decoder can determine that no color transform is usedfor a current CU when a syntax element(s) indicating different bitdepths for different color components is received.

In an embodiment, when transform skip flags signaled in TU level (or CUlevel) for each color component index for a current CU are all equal to1 or 0, a QP for each component of the current CU is equal to 4, and thebit depths of the luma and chroma components are different, colortransform (in spatial residual domain or before prediction) is not usedfor the CU. In an example, the transform skip flags for the luma or thechroma component specifies whether a transform (e.g., DCT transform) isapplied to an associated transform block or not. The transform skip flagequal to 1 specifies that no transform is applied to the associatedtransform block. The transform skip flag equal to 0 specifies that thedecision whether transform is applied to the associated transform blockor not depends on other syntax elements.

In an example, when the transform skip flag is not present, it isinferred as follows. If a block-based differential pulse code modulation(BDPCM) flag of the respective color component is equal to 1, thetransform skip flag is inferred to be equal to 1. Otherwise (the BDPCMflag is equal to 0), the transform skip flag is inferred to be equal to0. For example, the BDPCM flag equal to 1 specifies that BDPCM isapplied to the current chroma or luma coding blocks i.e. the transformis skipped. The BDPCM equal to 0 specifies that BDPCM is not applied tothe current chroma or luma coding blocks. In an example, when the BDPCMflag is not present, the BDPCM flag is inferred to be equal to 0.

In an example, when the QP for each component of the CU is equal to 4, acorresponding quantization step for quantizing transform coefficientscan have a value of 1. As a result, there are no changes to values ofthe original transform coefficients after the quantization operation.Accordingly, the QP of a value of 4 has the effect of a lossless mode.

In an embodiment, when transform skip flags signaled in TU level (or CUlevel) for each color component index for a current or CU are all equalto 1 or 0 and a QP for each component of the current CU is equal to 4,color transform (in spatial residual domain or before prediction) is notused for the current CU.

In an embodiment, when transform skip flags signaled in TU level (or CUlevel) for each color component index for a current CU are all equal to1 or 0, color transform (in spatial residual domain or beforeprediction) is not used for the current CU.

In some embodiments, two options of color space conversion are providedfor encoding or decoding processing. An encoder can select one of thetwo options to perform a color transform, for example, based on arate-distortion cost evaluation, and signal the selection in abitstream. Or, an encoder can signal to a decoder (e.g., by transmittinga flag) that a color transform is applied for a CU. The decoder candetermine which one of the two options is used for the CU based on a setof conditions. Information of those conditions can be received orderived from one or more syntax elements transmitted in a bitstream.

In some examples, inverse color space conversions of the two options ofcolor transform can be defined as follows:

Option 1:tmp=rY[x][y]−(rCb[x][y]>>1)  Eq. (17):rY[x][y]=tmp+rCb[x][y]  Eq. (18):rCb[x][y]=tmp−(rCr[x][y]>>1)  Eq. (19):rCr[x][y]=rCb[x][y]+rCr[x][y]  Eq. (20):

Option 2:tmp=rY[x][y]−rCb[x][y]  Eq. (21):rY[x][y]=rY[x][y]+rCb[x][y]  Eq. (22):rCb[x][y]=tmp−rCr[x][y]  Eq. (23):rCr[x][y]−tmp+rCr[x][y]  Eq. (24):where the inputs to each color transform option can be arrays ofresidual samples: an array of luma residual samples of a luma residualblock, denoted by rY, an array of chroma residual samples of a firstchroma residual block, denoted by rCb, and an array of chroma residualsamples of a second chroma residual block, denoted by rCr. [x][y]represents a coordinate of the residual samples within the respectiveresidual block. For example, x can be an integer in a range from 0 to ablock width minus 1, and y can be an integer in a range from 0 to ablock height minus 1.

For example, the two options of color transform, Option 1 and Option 2,can be used at the inverse color-space transform unit (1210) in the FIG.12 example to convert luma and chroma residual blocks to a modifiedversion of luma and chroma residual blocks.

According to the present disclosure, the color transform correspondingto the first option, Option 1, can be suitable for lossless processingof CUs, while the color transform corresponding to the second option,Option 2, can be suitable for lossy processing of CUs. For example, forCUs with a lossless mode enabled (e.g., transform and quantization skipflag equal to 1 is signaled or interpreted), Option 1 color transformcan have a better decorrelation effect than Option 2 color transform.

In an embodiment, for a current CU, when color transform is applied andsignaled on CU level, transform skip flags signaled in CU level (or TUlevel) for each component index are all equal to 1, and a QP of eachcomponent of the current CU is equal to 4, it can be determined than theOption 1 color transform can be applied for the CU. For example, a CUlevel color transform enabled flag can be received from a bitstream at adecoder. The color transform enabled flag equal to 1 specifies that thedecoded residuals of the current coding unit are applied using a colorspace conversion. The color transform enabled flag equal to 0 specifiesthat the decoded residuals of the current coding unit are processedwithout a color space conversion. In an embodiment, when the colortransform enabled flag is not present, the color transform enabled flagis inferred to be equal to 0. One or more syntax elements indicating theQP of value 4 can also be received from the bitstream. Based on valuesof the CU level color transform enabled flag, the transform skip flags,and the QP value, the decoder can determine that Option 1 colortransform is applied to the current CU.

In an embodiment, for a current CU, when color transform is applied andsignaled on CU level, and at least one of transform skip flags signaledin CU level (TU level) for each component index is not equal to 1, itcan be determined that the Option 2 color transform is applied to thecurrent CU. In an embodiment, for a current CU, when color transform isapplied and signaled on CU level, and a QP of one of the components ofthe current CU is not equal to 4, it can be determined that the Option 2color transform is applied to the current CU.

In an embodiment, for a current CU, when color transform is applied andsignaled in CU level and transform skip flags signaled in CU level foreach component index are all equal to 1, it can be determined that theOption 1 color transform is applied to the current CU.

In an embodiment, only Option 1 color transform is applied for a CTU, aslice, a video sequence, or the like. Option 2 color transform is notused. In an embodiment, only Option 1 color transform is applied for aCTU, a slice, a video sequence, or the like regardless of values oftransform and quantization bypass flags. In an example, the transformand quantization bypass equal 1 specifies transform and quantization isbypassed for a current CU. The current CU thus can be processed in alossless mode. The transform and quantization bypass equal 1 specifiestransform and quantization is not bypassed for the current CU, orwhether the transform and quantization is bypassed for the current CUdepends on other information.

In an embodiment, the selection of color transform (whether Option 1 orOption 2) depends on a transform skip flag(s) and/or a BDPCM flag(s) ofa current CU. In an example, the selection of the color transform optioncan depend on transform skip flags of each color component of thecurrent CU as described above.

In an example, the selection of a color transform option can depend on aBDPCM flag of the current CU. For example, when the BDPCM flag indicatesa BDPCM mode is applied to a luma or chroma component of the current CU,it can be determined that transform processing is skipped for therespective luma or chroma component. Otherwise, when the BDPCM flagindicates a BDPCM mode is not applied to a luma or chroma component ofthe current CU, it can be determined that transform is applied for therespective luma or chroma component. The transform skip flags canaccordingly be interpreted to be zero or one. Accordingly, the selectionof the color transform option, as described above, can depend on thetransform skip flags of each color component with the interpretedvalues.

In an example, a BDPCM mode is used for processing a current CU. In aprediction process, for a current pixel X having pixel A as leftneighbor, pixel B as top neighbor and C as top-left neighbor, aprediction P(X) can be determined by

${P(X)} = \left\lbrack \begin{matrix}{{{\min\left( {A,B} \right)}\mspace{14mu}{if}\mspace{14mu} C} \geq {\max\left( {A,B} \right)}} \\{{{\max\left( {A,B} \right)}\mspace{14mu}{if}\mspace{14mu} C} \leq {\min\left( {A,B} \right)}} \\{A + B - {C\mspace{14mu}{otherwise}}}\end{matrix} \right.$The prediction process uses unfiltered reference pixels when predictingthe top row and left column of the current CU. The prediction processthen uses reconstructed pixels for the rest of the current CU. Pixelsare processed in raster-scan order inside the CU. The prediction errorscan then be quantized in spatial domain without transform followed byentropy coding. In an example, a BDPCM flag can be transmitted in CUlevel to indicate whether a regular intra coding or a BDPCM is used.

X. Examples of Decoding Process with Color Transform

FIG. 19 illustrates an embodiment of a process (1900) performed by adecoder such as decoder (710). The process (1900) can start from(S1901), and proceed to (S1910).

At (S1910), a syntax element can be received from a bitstream of a codedvideo. For example, the syntax element can be a CU level color transformenabled flag indicating that residual blocks of a current CU areprocessed with a color space conversion. The residual blocks of thecurrent CU can include a luma residual block, a first chroma residualblock, and a second chroma residual block.

At (S1920), in response to receiving the syntax element indicating thatthe residual blocks of the current CU are processed with the color spaceconversion, one of two options of color space conversion equations canbe selected. In an example, the two options of color space conversionequations can correspond to the Option 1 and Option 2 color spaceconversion represented by the equations (17)-(20) and equations(21)-(24). In other examples, the two options of color space conversioncan take other forms corresponding to other color transform equations.

In an example, one of two options of color space conversion equationscan be selected according to a value of a transform skip flagcorresponding to each of the residual blocks of the current CU. Thetransform skip flags can be received from the bit stream of the codedvideo. Or, values of the transform skip flags can be interpreted basedon other information if the transform skip flags are not signaled in thebit stream of the coded video.

In an example, one of two options of color space conversion equationscan be selected according a QP of each of the residual blocks of thecurrent CU. For example, when the QP has a value of 4, and the transformskip flags of each residual blocks are equal to 1, the Option 1 colortransform can be selected. When the QP has a value not equal to 4, theOption 2 color transform can be selected.

In an example, the selection of one of the two options of colortransform can depends on a BDPCM flag of the current CU. For example,when the BDPCM flag indicate a BDPCM mode is applied to a colorcomponent of the current CU, the transform skip flag of this colorcomponent of the current CU can be interpreted to have a value of 1. Theselection of one of the two options can be based on the value of thetransform skip flag in combination with values of transform skip flagsof other color components of the current CU.

At (S1930), an inverse color space conversion using the selected optionof color space conversion equations can be applied to the residualblocks of the current CU to generate modified versions of the residualblocks of the current CU. The process (1900) can proceed to (S1999) andterminate and (S1999).

XI. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 20 shows a computersystem (2000) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 20 for computer system (2000) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (2000).

Computer system (2000) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (2001), mouse (2002), trackpad (2003), touchscreen (2010), data-glove (not shown), joystick (2005), microphone(2006), scanner (2007), camera (2008).

Computer system (2000) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (2010), data-glove (not shown), or joystick (2005), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (2009), headphones(not depicted)), visual output devices (such as screens (2010) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (2000) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(2020) with CD/DVD or the like media (2021), thumb-drive (2022),removable hard drive or solid state drive (2023), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (2000) can also include an interface (2054) to one ormore communication networks (2055). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CANBus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general purpose data ports or peripheral buses (2049) (such as,for example USB ports of the computer system (2000)); others arecommonly integrated into the core of the computer system (2000) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (2000) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (2040) of thecomputer system (2000).

The core (2040) can include one or more Central Processing Units (CPU)(2041), Graphics Processing Units (GPU) (2042), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(2043), hardware accelerators for certain tasks (2044), graphicsadapters (2050), and so forth. These devices, along with Read-onlymemory (ROM) (2045), Random-access memory (2046), internal mass storagesuch as internal non-user accessible hard drives, SSDs, and the like(2047), may be connected through a system bus (2048). In some computersystems, the system bus (2048) can be accessible in the form of one ormore physical plugs to enable extensions by additional CPUs, GPU, andthe like. The peripheral devices can be attached either directly to thecore's system bus (2048), or through a peripheral bus (2049). In anexample, the screen (˜˜×10) can be connected to the graphics adapter(˜˜×50). Architectures for a peripheral bus include PCI, USB, and thelike.

CPUs (2041), GPUs (2042), FPGAs (2043), and accelerators (2044) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(2045) or RAM (2046). Transitional data can also be stored in RAM(2046), whereas permanent data can be stored for example, in theinternal mass storage (2047). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (2041), GPU (2042), massstorage (2047), ROM (2045), RAM (2046), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (2000), and specifically the core (2040) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (2040) that are of non-transitorynature, such as core-internal mass storage (2047) or ROM (2045). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (2040). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(2040) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (2046) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (2044)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

ASIC: Application-Specific Integrated Circuit

BMS: Benchmark Set

CANBus: Controller Area Network Bus

CBF: Coded block flag

CCLM: Cross-component Linear Model

CD: Compact Disc

CPUs: Central Processing Units

CRT: Cathode Ray Tube

CTBs: Coding Tree Blocks

CTUs: Coding Tree Units

CU: Coding Unit

DT: Dual Tree

DVD: Digital Video Disc

FPGA: Field Programmable Gate Areas

GOPs: Groups of Pictures

GPUs: Graphics Processing Units

GSM: Global System for Mobile communications

HDR: high dynamic range

HEVC: High Efficiency Video Coding

HRD: Hypothetical Reference Decoder

IC: Integrated Circuit

ISP: Intra Sub-Partitions

JVET: Joint Video Exploration Team

LAN: Local Area Network

LCD: Liquid-Crystal Display

LTE: Long-Term Evolution

MPM: most probable mode

MV: Motion Vector

JEM: joint exploration model

OLED: Organic Light-Emitting Diode

PBs: Prediction Blocks

PCI: Peripheral Component Interconnect

PLD: Programmable Logic Device

PU: Prediction Unit

QP: Quantization Parameter

RAM: Random Access Memory

ROM: Read-Only Memory

SBT: Sub-block transform

SDR: standard dynamic range

SEI: Supplementary Enhancement Information

SNR: Signal Noise Ratio

SSD: Solid-State Drive

TU: Transform Unit

USB: Universal Serial Bus

VUI: Video Usability Information

VVC: Versatile Video Coding

WAIP: Wide-Angle Intra Prediction

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method of video decoding performed in a video decoder, comprising: receiving a syntax element from a bitstream of a coded video indicating whether residual blocks of a current coding unit (CU) are processed with a color space conversion, the residual blocks of the current CU including a luma residual block, a first chroma residual block, and a second chroma residual block; in response to receiving the syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, applying, based on a transform skip flag corresponding to each of the residual blocks of the current CU having a value of 1 and a quantization parameter (QP) corresponding to each of the residual blocks of the current CU having a value of 4, an inverse color space conversion using a first option of color space conversion equations to the residual blocks of the current CU to generate modified versions of the residual blocks of the current CU, wherein the first option of the color space conversion equations includes: tmp=rY[x][y]−(rCb[x][y]>>1), rY[x][y]=tmp+rCb[x][y], rCb[x][y]−tmp−(rCr[x][y]>>1), and rCr[x][y]=rCb[x][y]+rCr[x][y]; and inputs to the first option of the color space conversion equations include: an array of luma residual samples of the luma residual block with elements rY[x][y], an array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and an array of chroma residual samples of the second chroma residual block with elements rCr[x][y]; and outputs from the first option of the color space conversion equations include: a modified array of luma residual samples of the luma residual block with elements rY[x][y], a modified array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and a modified array of chroma residual samples of the second chroma residual block with elements rCr[x][y].
 2. The method of claim 1, further comprising: in response to the first syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, selecting one of two options of color space conversion equations, wherein the two options of color space conversion equations include: the first option of color space conversion equations, and a second option of color space conversion equations including: tmp=rY[x][y]−(rCb[x][y]>>1), rY[x][y]=tmp+rCb[x][y], rCb[x][y]−tmp−(rCr[x][y]>>1), and rCr[x][y]=rCb[x][y]+rCr[x][y]; and inputs to the second option of the color space conversion equations include: an array of luma residual samples of the luma residual block with elements rY[x][y], an array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and an array of chroma residual samples of the second chroma residual block with elements rCr[x][y]; and outputs from the second option of color space conversion equations include: a modified array of luma residual samples of the luma residual block with elements rY[x][y], a modified array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and a modified array of chroma residual samples of the second chroma residual block with elements rCr[x][y].
 3. The method of claim 2, wherein the selecting one of the two options of the color space conversion equations includes: selecting the second option of the color space conversion equations when at least one of transform skip flags corresponding to the residual blocks of the current CU has a value of
 0. 4. The method of claim 2, wherein the selecting one of the two options of the color space conversion equations includes: selecting the second option of the color space conversion equations when at least one of the QPs corresponding to the residual blocks of the current CU has a value not equal to
 4. 5. The method of claim 2, further comprising: receiving a second syntax element indicating only the first option of color space conversion equations is applied, the second syntax element is one of a slice level syntax element, a picture level syntax element, or a sequence level syntax element.
 6. The method of claim 2, further comprising: receiving a second syntax element indicating only the first option of color space conversion equations is applied irrespective of a value of a flag indicating whether transform and quantization are bypassed for residuals of a CU, the second syntax element is one of a slice level syntax element, a picture level syntax element, or a sequence level syntax element.
 7. The method of claim 2, further comprising: selecting one of the two options of the color space conversion equations for processing a CU according to a variable indicating whether a block-based differential pulse code modulation (BDPCM) is applied to the CU.
 8. The method of claim 1, further comprising: determining no color space conversion is applied to residual blocks of a CU when a bit depth of luma samples of the CU is different from a bit depth of chroma samples of the CU.
 9. The method of claim 1, further comprising: determining no color space conversion is applied to residual blocks of a CU when a bit depth of luma samples of the CU is different from a bit depth of chroma samples of the CU, and a transform skip flag corresponding to each of the residual blocks of the CU has a value of 1 or
 0. 10. The method of claim 1, further comprising: determining no color space conversion is applied to residual blocks of a CU when a bit depth of luma samples of the CU is different from a bit depth of chroma samples of the CU, the a QP corresponding to each of residual blocks of the CU has a value of 4, and a transform skip flag corresponding to each of the residual blocks of the CU has a value of 1 or
 0. 11. An apparatus of video decoding, comprising circuitry configured to: receive a first syntax element from a bitstream of a coded video indicating whether residual blocks of a current coding unit (CU) are processed with a color space conversion, the residual blocks of the current CU including a luma residual block, a first chroma residual block, and a second chroma residual block; in response to the first syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, apply, based on a transform skip flag corresponding to each of the residual blocks of the current CU having a value of 1 and a quantization parameter (QP) corresponding to each of the residual blocks of the current CU having a value of 4, an inverse color space conversion using a first option of color space conversion equations to the residual blocks of the current CU to generate modified versions of the residual blocks of the current CU, wherein the first option of the color space conversion equations includes: tmp=rY[x][y]−(rCb[x][y]>>1), rY[x][y]=tmp+rCb[x][y], rCb[x][y]−tmp−(rCr[x][y]>>1), and rCr[x][y]=rCb[x][y]+rCr[x][y]; and inputs to the first option of the color space conversion equations include: an array of luma residual samples of the luma residual block with elements rY[x][y], an array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and an array of chroma residual samples of the second chroma residual block with elements rCr[x][y]; and outputs from the first option of the color space conversion equations include: a modified array of luma residual samples of the luma residual block with elements rY[x][y], a modified array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and a modified array of chroma residual samples of the second chroma residual block with elements rCr[x][y].
 12. The apparatus of claim 11, wherein the circuitry is further configured to: in response to the first syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, select one of two options of color space conversion equations, wherein the two options of color space conversion equations include: the first option of color space conversion equations, and a second option of color space conversion equations including: tmp=rY[x][y]−rCb[x][y], rY[x][y]=rY[x][y]+rCb[x][y], rCb[x][y]=tmp−rCr[x][y], and rCr[x][y]=tmp+rCr[x][y], inputs to the second option of the color space conversion equations include: an array of luma residual samples of the luma residual block with elements rY[x][y], an array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and an array of chroma residual samples of the second chroma residual block with elements rCr[x][y]; and outputs from the second option of color space conversion equations include: a modified array of hum residual samples of the luma residual block with elements rY[x][y], a modified array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and a modified array of chroma residual samples of the second chroma residual block with elements rCr[x][y].
 13. The apparatus of claim 12, wherein the circuitry is further configured to: select the second option of the color space conversion equations when at least one of transform skip flags corresponding to the residual blocks of the current CU has a value of
 0. 14. The apparatus of claim 12, wherein the circuitry is further configured to: select the second option of the color space conversion equations when at least one of the QPs corresponding to the residual blocks of the current CU has a value not equal to
 4. 15. A non-transitory computer-readable medium storing instructions that, when executed by a processor, cause the processor to perform a method of video decoding, the method comprising: receiving a first syntax element from a bitstream of a coded video indicating whether residual blocks of a current coding unit (CU) are processed with a color space conversion, the residual blocks of the current CU including a luma residual block, a first chroma residual block, and a second chroma residual block; in response to the first syntax element indicating that the residual blocks of the current CU are processed with the color space conversion, applying, based on a transform skip flan corresponding to each of the residual blocks of the current CU having a value of 1 and a quantization parameter (QP) corresponding to each of the residual blocks of the current CU having a value of 4, an inverse color space conversion using a first option of color space conversion equations to the residual blocks of the current CU to generate modified versions of the residual blocks of the current CU, wherein, tmp=rY[x][y]−(rCb[x][y]>>1), rY[x][y]=tmp+rCb[x][y], rCb[x][y]−tmp−(rCr[x][y]>>1), and rCr[x][y]=rCb[x][y]+rCr[x][y]; and inputs to the first option of the color space conversion equations include: an array of luma residual samples of the luma residual block with elements rY[x][y], an array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and an array of chroma residual samples of the second chroma residual block with elements rCr[x][y]; and outputs from the first option of the color space conversion equations include: a modified array of luma residual samples of the luma residual block with elements rY[x][y], a modified array of chroma residual samples of the first chroma residual block with elements rCb[x][y], and a modified array of chroma residual samples of the second chroma residual block with elements rCr[x][y]. 